Private-equity firms are expanding in U.S. energy infrastructure, as oil-and-gas exports rise and companies that gather, process and transport the fuels see their traditional sources of capital dry up, a new report said.

The increased participation of private equity, the opening of energy export markets and the increasing demands from investors that these midstream companies fund their own growth are upending the energy-infrastructure sector, according to research from Bain & Co.

“One key theme here is that private-equity firms are employing a much broader set of participation strategies,” said Whit Keuer, a Bain partner in Dallas who is a member of the consulting firm’s energy and natural resources practice. He is also a co-author of “Three Trends Reshuffling North America’s Midstream Industry,” a report being released today.

“The focus historically was more around project developers, and then in the last two or three years you’ve seen them taking more of an investor role.”

Private-equity firms these days are forming joint ventures with midstream energy companies more frequently, as traditional sources of capital step back from the sector. North American midstream businesses raised an average of $67 billion in debt and equity capital annually between 2012 and 2018, but that volume is expected to drop to an average of about $25 billion this year through 2023, the report said.

Many recent transactions illustrate the changing role of private equity in the energy sector. Some have shown how firms are expanding into midstream niches such as wastewater management and export-related infrastructure.

Solaris Water Midstream LLC said last week it had formed a joint venture with Concho Resources Inc. to manage wastewater byproducts of the energy company’s oil-and-gas operations in New Mexico’s Northern Delaware Basin. Solaris is backed by private-equity firms Trilantic Capital Partners, Yorktown Partners and Solaris Energy Capital.

Private-equity firms also are purchasing minority stakes in public midstream companies or taking some of them private, the report said. One such deal is IFM Investors’ buyout of Buckeye Partners LP. IFM said the May deal placed an enterprise value of $10.3 billion on the master limited partnership, which operates pipelines and storage terminals. Bain said it had identified more than a dozen publicly traded midstream companies that may be good candidates for buyouts.

Finally, the report said private-equity capital can be instrumental in the expected consolidation of the sector. One way firms are promoting such consolidation is by acquiring noncore assets that energy companies are selling to raise capital. Another is through backing outright mergers.

“We just think there’s a strong logic for M&A right now,” Mr. Keuer said. “The industry is very fragmented. There’s a lot of value in combining companies in basin to get to greater operational efficiencies.”

The report added, however, that firms need to do a better job of devising plans to improve the businesses they acquire, especially considering the premiums they are paying in their transactions, as much as 20% above public valuations.

Firms need to better evaluate potential production growth within a target company’s area and assess how much of it the business can capture. Investors also need to look at contract durations, define which assets are core and noncore, and explore opportunities to improve procurement, maintenance and plant utilization, the report said.

“Our observation is that private-equity firms don’t do enough on really sharpening their pencil or getting more granular around what is your strategic and operational turnaround plan,” Mr. Keuer said. “In a world with a lot of capital chasing the deals, you’re going to have to have that in order to be able to generate the returns.”